In a plasma processing apparatus, a process such as a film formation or an etching that is accompanied with chemical reaction is performed. During performing the process, in order to generate the uniform plasma throughout an entire surface of the object to be processed, a density of a gas required to perform the process and a density of a reaction gas generated due to the chemical reaction should be constant. The gas is introduced into the processing chamber through a plurality of gas discharging holes that are formed with substantially regular pitches on a surface facing the object to be processed (for example, refer to Patent Document 1).
However, even when the plurality of gas discharging holes are formed with regular pitches, flow amount of the gas discharged from the gas discharging hole that is close to a gas supply source may be greater than flow amount of the gas discharged from the gas discharging hole that is far from the gas supply source. For example, when the gas is distributed from a main gas passage to a plurality of branch gas passages, if a gas conductance of the main gas passage is not sufficiently greater than the gas conductance of the branch gas passages, a sufficient amount of gas is discharged from the gas discharging hole formed on a branch gas passage that is close to the gas supply source, while the amount of gas reaching the branch gas passage that is far from the gas supply source is small, and thus, the branch gas passage that is far from the gas supply source may not discharge a sufficient amount of gas. Thus, the density of gas in the processing chamber is not uniform, and thus, the plasma may be generated nonuniformly. To avoid the above phenomenon, a gas supplying path should be designed so that the gas conductance of the main gas passage is sufficiently greater than the gas conductance of the plurality of branch gas passages, and thus, the gas may be distributed evenly from the main gas passage to the branch gas passages.    (Patent Document 1) Japanese Laid-open Patent Publication No. hei 2-114530